Re: [PATCH v1 net] kcm: Serialise kcm_sendmsg() for the same socket.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Eric Dumazet <edumazet@google.com>
Date: Mon, 19 Aug 2024 17:56:55 +0200
> On Fri, Aug 16, 2024 at 12:04 AM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > syzkaller reported UAF in kcm_release(). [0]
> >
> > The scenario is
> >
> >   1. Thread A builds a skb with MSG_MORE and sets kcm->seq_skb.
> >
> >   2. Thread A resumes building skb from kcm->seq_skb but is blocked
> >      by sk_stream_wait_memory()
> >
> >   3. Thread B calls sendmsg() concurrently, finishes building kcm->seq_skb
> >      and puts the skb to the write queue
> >
> >   4. Thread A faces an error and finally frees skb that is already in the
> >      write queue
> >
> >   5. kcm_release() does double-free the skb in the write queue
> >
> > When a thread is building a MSG_MORE skb, another thread must not touch it.
> >
> > Let's add a per-sk mutex and serialise kcm_sendmsg().
> >
> >
> > Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> > Reported-by: syzbot+b72d86aa5df17ce74c60@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=b72d86aa5df17ce74c60
> > Tested-by: syzbot+b72d86aa5df17ce74c60@syzkaller.appspotmail.com
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> 
> Reviewed-by: Eric Dumazet <edumazet@google.com>
> 
> I wonder if anyone is using KCM....

Same impression :)
Maybe it's time to remove KCM.
https://lore.kernel.org/netdev/CALx6S37hSfQWw3Rku8vsavn8ejk0fndRk+=-=73gU7G-RbnK8Q@mail.gmail.com/